1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units that are formed by sandwiching an electrode assembly between separators.
2. Description of the Related Art
Among fuel cell units, there is one type that is formed in a plate shape by sandwiching between a pair of separators an electrode assembly that is formed by placing an anode electrode and a cathode electrode respectively on either side of a solid polymer electrolyte membrane. A fuel cell is formed by stacking in the thickness direction of the fuel cell units a plurality of fuel cell units that are structured in this way.
In each fuel cell unit there are provided a communication path for fuel gas (for example, hydrogen) on one surface of the anode side separator that is positioned facing the anode electrode, and a communication path for oxidizing gas (for example, air that contains oxygen) on one surface of the cathode side separator that is positioned facing the cathode electrode. In addition, a communication path for a cooling medium (for example, pure water) is provided between adjacent separators of adjacent fuel cell units.
When fuel gas is supplied to the electrode reaction surface of the anode electrode, hydrogen is ionized here and moves to the cathode electrode via the solid polymer electrolyte membrane. Electrons generated between these two are extracted to an external circuit and used as direct current electrical energy. Because oxidizing gas is supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react to generate water. Because heat is generated when water is created at the electrode reaction surface, the electrode reaction surface is cooled by a cooling medium made to flow between the separators.
The fuel gas, oxidizing gas (generically known as reaction gas), and the cooling medium each need to flow through a separate communication path. Therefore, sealing technology that keeps each communication path sealed in a fluid-tight or airtight condition is essential.
Examples of portions that need to be sealed are: the peripheries of penetrating supply ports formed in order to supply and distribute reaction gas and cooling medium to each fuel cell unit of the fuel cell; the peripheries of discharge ports that collect and discharge the reaction gas and cooling medium that are discharged from each fuel cell unit; the outer peripheries of the electrode assemblies; and the outer peripheries between the separators of adjacent fuel cell units. A material that is soft yet also has the appropriate resiliency such as organic rubber is employed for the sealing member.
In recent years, however, size and weight reduction, as well as a reduction in the cost of fuel cells, have become the main barriers in progress towards the more widespread application of fuel cells through their being mounted in actual vehicles.
Methods that have been considered for reducing the size of a fuel cell include making each fuel cell unit forming the fuel cell thinner, more specifically, reducing the size of the space between separators while maintaining a maximum size for the reaction gas communication path formed inside each fuel cell unit; and also making the separators thinner.
However, a limit is imposed on how thin the separators can be made by the strength requirements for each separator and by the rigidity requirements for the fuel cell. Reducing the height of the sealing members is effective in reducing the size of the spacing between separators, however, the height of the sealing members needs to be sufficient for the sealing members to be able to be pressed down enough to ensure the required sealing performance is obtained. Therefore, there is also a limit to how much the height of the sealing members can be reduced.
Furthermore, in a fuel cell unit, although the volume occupied by the sealing members is indispensable in order for the reaction gas and cooling medium to be sealed in, because this space contributes substantially nothing to power generation it needs to be made as small as possible.
FIG. 23 is a plan view showing a conventional fuel cell. In FIG. 23 the symbol 107 indicates a communication port such as a fuel gas supply port and discharge port, an oxidizing gas supply port and discharge port, and a cooling medium supply port and discharge port that each penetrate the fuel cell 106 in the direction in which separators 109 and 110 are stacked. The symbol 112 indicates an area formed by a plurality of fuel gas communication paths, oxidizing gas communication paths, and cooling medium communication paths running along the separators 109 and 110.
FIG. 24 is a longitudinal cross-sectional view of a conventional fuel cell 106 taken along the line Xxe2x80x94X in FIG. 23. As can be seen in plan view, in order to make the volume occupied by the sealing member (which doesn""t contribute to power generation) as small as possible, conventionally, by locating gas sealing members 102 and 103, which respectively seal a fuel gas communication path 100 and an oxidizing gas communication path 101, together with a cooling surface sealing member 104, which seals a cooling medium communication path, aligned in a row in the stacking direction of the fuel cell units 105, the outer dimensions in the stacking direction of the fuel cell 106 are kept to the minimum.
However, the drawback with the fuel cell 106 that is structured in this manner is that if the gas sealing members 102 and 103 that seal the communication paths 100 and 101 as well as the cooling surface sealing member 104 are all placed in a row in the stacking direction of the fuel cell unit 105, then the thickness of the fuel cell 106 cannot be made less than a value obtained by adding the height of the cooling surface sealing member 104 to the minimum thickness of each fuel cell unit 105, and multiplying this result by the number of fuel cell units stacked in the fuel cell.
In order to explain this more specifically, the description will return to FIG. 24. FIG. 24 is a longitudinal cross-sectional view showing a cross section of the fuel cell 106 in the vicinity of the fuel gas supply port 107 in the stacking direction of the fuel cell units 105. According to FIG. 24, the fuel gas supply port 107 and the fuel gas communication path 100 that are isolated in a sealed state by the gas sealing members 102 and 103 are connected by a communication path 108. The communication path 108 is provided in the separator 109 so as to detour around, in the thickness direction of the separator 109, the gas sealing member 102 that seals the entire periphery of the fuel gas communication path 100. Moreover, the separator 110 also has a similar communication path (not shown) in the oxidizing gas supply port (not shown).
Accordingly, each of the separators 109 and 110 are formed comparatively thickly in order to form the communication path 108, however, as is seen in the cross section in FIG. 24, at the position of the seal line where each of the sealing members 102 to 104 are placed, the separators 109 and 110 are formed with the minimum thickness needed to ensure the required strength, and it is not possible to make them any thinner.
Moreover, because each of the sealing members 102 to 104 is formed with the minimum height needed to secure the sealing performance, it is not possible to reduce the height of the sealing members 102 to 104 any further.
As a result, although the thickness of the fuel cell 106 is found by multiplying the number of stacks by the sum of the minimum thickness of the two separators 109 and 110, the thickness needed to form the communication path 108, the height of the two gas sealing members 102 and 103, the thickness of the solid polymer electrolyte membrane 111, and the height of the cooling surface sealing member 104, because these are all indispensable it is extremely difficult to achieve any further reduction in thickness.
The present invention was conceived in view of the above circumstances, and it is an object thereof to provide a fuel cell that has been made lighter and smaller by reducing the thickness thereof in the stacking direction, while reliably sealing the respective communication paths using the respective sealing members between the separators and the electrode assemblies that form the fuel cell.
In order to solve the above problems, a first aspect of the present invention is a fuel cell comprising fuel cell units, the fuel cell units being stacked and having at least one cooling medium flow path therebetween, and the cooling medium flow path sealed by a cooling surface sealing member, each fuel cell unit comprising: an electrode assembly formed by disposing electrodes on both sides of an electrolyte; separators that sandwich the electrode assembly in the thickness direction thereof; and gas sealing members that are disposed at an outer peripheral portion of the electrode assembly, and that seal respective reaction gas passages that are formed between each separator and the electrode assembly and are bounded by the separators and electrode assembly, wherein in each of the separators there are provided reaction gas communication ports and cooling medium communication ports that are provided on the outer side of electrode assembly sealing members of the gas sealing members, and reaction gas communication paths that detour around the electrode assembly sealing members of the gas sealing members in the thickness direction of the separators and connect reaction gas communication ports with reaction gas passages; and in at least one separator of the separators that are disposed adjacent to each other and have the cooling medium flow path therebetween, there is provided a convex portion that protrudes from a rear surface of the reaction gas communication paths over at least an area that corresponds to the reaction gas communication paths, and in the other separator there is provided a concave portion that receives the convex portion.
According to the fuel cell of the present invention, because a convex portion is provided in one separator it is possible to reduce the thickness of this separator to the minimum, and to secure the thickness needed to form the reaction gas communication paths. Moreover, because a concave portion that receives the convex portion is provided in the other separator, it is possible to use the concave portion to cancel out the increase in thickness created by the convex portion. Accordingly, the thickness necessary to form the reaction gas communication paths may be secured by one separator and the other separator together, and it is not necessary to secure the thickness necessary to form the reaction gas communication paths in each one of both separators. Therefore, it is possible to reduce the necessary thickness of each separator by the corresponding amount, and to reduce the thickness of each fuel cell unit. Note that, if the thickness required to form the reaction gas communication paths is secured in one separator of the pair of separators, then it is possible to form the reaction gas communication path without providing the convex portion. Accordingly, because it is not necessary to provide the concave portion corresponding to the convex portion in the other separator, the thickness of the other separator can be reduced to the minimum. By employing such a structure, because the combined thickness of both separators can be maintained at the minimum value, and because the number of locations where such portions are formed is reduced by half compared with when a convex portion and concave portion are provided in both separators, the manufacturing process is simplified.
Because as many as several hundred fuel cell units may be stacked in a fuel cell, it is possible to achieve a marked size reduction in the fuel cell as a whole in accordance with how many unit cells, each of whose thickness has been reduced, are stacked.
Another aspect of the present invention is a fuel cell in which the cooling surface sealing member that seals the cooling medium communication path from the reaction gas communication ports is provided at a position that is closer to the reaction gas communication port relative to the reaction gas communication paths.
According to the fuel cell of this aspect of the invention, because the cooling surface sealing member that seals the cooling medium communication path is provided at a position that is shifted towards the communication port side from the communication paths, the position of the cooling surface sealing member in the stacking direction can be decided irrespectively of the communication paths provided in the vicinity of the gas sealing member. As a result, it is possible to overlap the position of the cooling surface sealing member with the position of the communication paths in the stacking direction of the fuel cell units. It is therefore possible to reduce the thickness of each fuel cell unit by the amount of this overlapping portion.
Yet another aspect of the present invention is a fuel cell in which, other than in the vicinity of the reaction gas communication path, the cooling surface sealing member is placed at substantially the same position as the gas sealing members as seen from a stacking direction.
According to the fuel cell of this aspect of the invention, in the vicinity of a communication path that is formed so as to detour around the gas sealing member in the thickness direction, a separator must be made thicker by the amount of the height of the communication path, therefore, the cooling surface sealing member is shifted towards the communication ports and a situation in which the gas sealing members are placed on the same seal line as the cooling surface sealing members is avoided. In portions other than the vicinity of the communication path, by placing the gas sealing members aligned in a row with the cooling surface sealing members in the stacking direction of the fuel cell units, it is possible to reduce the surface area of the fuel cell units. Moreover, by shifting the seal line in the vicinity of the communication ports, in the same way as for the fuel cell according to the above first aspect, it is possible to overlap the cooling surface sealing member with the communication paths in the stacking direction, and the cooling surface sealing member and the gas sealing member can be placed adjacent to each other in the stacking direction, thereby allowing the thickness of the fuel cell to be reduced.
In the fuel cell of the present invention, the cooling medium communication path may be formed in every space between the fuel cell units adjacent to each other.
In the fuel cell of the present invention, two or more fuel cell units may form a set of fuel cell units, and the cooling medium communication path may be formed in every space between adjacent sets of fuel cell units.
In the fuel cell of the present invention, the gas sealing member may be provided as a double sealing structure on one side of one separator forming the fuel cell unit.
In the fuel cell of the present invention, one electrode of the electrodes may extend to a size that is substantially the same as that of the electrolyte.
In the fuel cell of the present invention, the separators may be formed of metal plates by press forming.